Various gene therapy approaches have been developed as alternatives to traditional protein therapy approaches. However, important challenges still remain in gene therapy. One of the major challenges of gene therapy is to achieve efficient influx of genes across plasma membranes (in animal cells) and nuclear membranes with minimal cytotoxicity.
Gene therapy systems can be broadly classified into viral vector-mediated systems and nonviral vector-mediated systems. Viral vectors are constructed using retroviruses or adenoviruses and have the advantage of high transfection efficiency into cells. However, viral vectors have problems associated with in vivo immunogenicity and suffer from inherent problems associated with genetic recombination. In attempts to overcome the stability problems of such viral vectors, various polymeric gene delivery systems have been developed as alternatives to traditional viral vector-based gene delivery strategies. For efficient gene delivery, polymeric vectors need to overcome intracellular trafficking barriers such as endosomal escape and nuclear localization.
Gene leakage occurs from synthetic peptide-based gene delivery systems in endosomal membranes at low pH, leading to DNA condensation and rapid endosomal escape. Accordingly, the use of synthetic peptide-based gene delivery systems can overcome the problems encountered with polymeric gene delivery systems.
For such reasons, a variety of synthetic peptides have been developed to promote in vitro gene delivery into several cell lines. However, these synthetic peptides also suffer from the problems of toxicity and serum instability in in vivo applications. Particularly, in this regard, research is being conducted on vectors using short cationic peptides. Even in this case, there are some problems, such as unstable nucleic acids in extracellular spaces. Other problems of the vectors are poor stability of complexes with nucleic acids and low gene expression level.
Particularly, in the case of nucleic acid delivery for RNAi, the stability of complexes with nucleic acids is low, making it difficult to deliver the nucleic acids into synthetic peptides. In view of this, lipids, liposomes, etc. have been used for nucleic acid delivery but failed to achieve effective in vivo systemic circulation.
Thus, the present inventors have investigated gene delivery systems using peptides, and as a result, found that PEGylation of a vector having a nona-arginine (R9) structure in which cysteine (Cys) residues are attached to one or both termini of the R9 can markedly improve the in vivo delivery efficiency of siRNA. The present invention has been accomplished based on this finding.